When you get stuck in the game, the three kinds of hints might help you out.
Lightbulb: reveals a random letter on the board.
Finger: reveals a letter on a specific blank space.
Lightning: reveals 5 random letters on the board.
The hints that you have are shown with the remaining number in the puzzle screen. You could simply click the hint buttons to use them. When the hints in your balance are used up, the next click of hints will consume the corresponding coins.
Besides, when you have a trouble with finding an answer, the free Shuffle button could also give you a new thinking by rearranging the letters.